1. Field of the Invention
The present invention relates to a Group III nitride semiconductor device.
2. Related Background Art
Patent Literatures 1 and 2 describe a method for forming an ohmic electrode on a p-type gallium nitride (GaN) layer grown on a (0001) plane or c-plane. In this method, Ni/Au film is evaporated onto the p-type GaN layer and thereafter a thermal treatment is carried out in an oxygen atmosphere. An oxide film exists on the surface of the p-type GaN layer, but nickel absorbs the oxide film by this method. Then nickel binds to oxygen atoms of the oxide film, thereby removing the oxide film. This allows gold to come into contact with the p-type GaN crystal, thereby forming an ohmic contact.
Patent Literature 3 describes the result of an experiment. In this experiment, a p-type Group III nitride semiconductor layer is grown on a sapphire substrate. Then the substrate is heated to 300° C. and thereafter platinum or nickel is evaporated onto a surface of the Group III nitride semiconductor layer. Thereafter, the contact resistance is measured.
In the method described in Patent Literature 4, platinum is evaporated onto a p-type GaN contact layer. Then a thermal treatment is carried out in the range of 500° C. to 600° C. in an oxygen atmosphere to form an alloy electrode.    Patent Literature 1: Japanese Unexamined Patent Application Publication No. 5-291621    Patent Literature 2: Japanese Unexamined Patent Application Publication No. 9-64337    Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2004-247323    Patent Literature 4: Japanese Unexamined Patent Application Publication No. 11-186605